Fix small typo in performance consideration docs

[cdelv]

Hi Flax team,

Here is a fix to a minor typo in the documentation of performance considerations for nnx.

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[vfdev-5]

@cdelv thanks for the PR, can you please sync your change in md file with the jupyter notebook following this guideline: https://flax-linen.readthedocs.io/en/latest/contributing.html#syncing-notebooks